As a preparation for the migration to the grpc-gateway/v2 library we
declare each service's REST annotations in its own file. This is
optional in the v1 library but mandatory in v2.
In order to be able to register the subservers with the root grpc server
before we have all dependencies available, we wrap them in an
GrpcHandler struct. This struct will initially hold an empty reference
to the subservers, which allows us to register with the GRPC server, and
later populate and create the subserver instance.
